Delegate

Delegate is a pointer to a method or a group of the methods.

public class PhotoProcessor
{
  public delegate void PhotoFilterHandler(Photo photo);

  public void Process(string path, PhotoFilterHandler handler)
  {
    var photo = Photo.Load(path);
    handler(photo);
    photo.save();
  }
}

There are 2 delegates in .NET (Action and Func).

• Action: It can point to the method which doesn't return a value.
• Func: It can point to the method which returns a value.

public class PhotoProcessor
{
  public void Process(string path, Action<Photo> handler)
  {
    var photo = Photo.Load(path);
    handler(photo);
    photo.save();
  }
}

Lambda expression

Func<int, int> square = num => num * num;

Event

Event is a design for communicating between classes.

There are 3 steps:

1. define a delegate
2. create an event based on the delegate
3. raise the event

public class VideoEncoder
{
  // 1. define a delegate
  public delegate void VideoEncodedEventHandler(object source, EventArgs args);

  // 2. create an event based on the delegate
  public event VideoEncodedEventHandler VideoEncoded;

  // 3. raise the event
  protected virtual void OnVideoEncoded()
  {
    if (VideoEncoded != null)
    {
      VideoEncoded(this, EventArgs.Empty);
    }
  }
}

The subscriber is:

public class MailService
{
  public void OnVideoEncoded(object source, EventArgs args)
  {
    // TODO
  }
}

// publisher
var videoEncoder = new VideoEncoder();
// subscriber
var mailService = new MailService();

// MailService subscribes the VideoEncoded event of videoEncoder.
videoEncoder.VideoEncoded += mailService.OnVideoEncoded;

To make it easier, we can use the delegate in .NET.

// public delegate void VideoEncodedEventHandler(object source, EventArgs args);
// public event VideoEncodedEventHandler VideoEncoded;
public event EventHandler VideoEncoded;

This is also a generic type. EventHandler<TEventArgs>.